Electrochemical gas analyzers of the type under consideration in this invention are well-known in the art and are exemplified by the disclosures in U.S. Pat. Nos. 3,767,552 and 4,077,861. These gas analyzers are in the form of electrochemical cells that are used as sensors for determining and electrically signalling the concentration of an electrochemically active gas in a gas mixture. The gas mixture may be air and since air includes oxygen, the sensing cell determines the oxygen concentration in the gas mixture or air. For this purpose, the electrochemical cell is generally defined with anode and cathode electrodes immersed in an electrolyte for producing an electrical output signal from the sensing cell representative of the quantity of concentration of the electrochemical active gas in the sensed gas mixture. These electrochemical cells as recognized in the prior art, are characterized as either galvanic cells or polarographic cells. Generally the galvanic cell results from the selection of an appropriate active anode material, such as lead, for causing a reaction at the cathode electrode with the gas undergoing analysis leading to the derivation of an electric output current from the cell representative of the concentration of the sensed gas and applied to a suitable sensing device connected to the sensing cell. A polarographic electrochemical cell results from the selection of silver, for example, as an anode electrode and applying a small polarizing voltage between the cathode and the anode electrodes to produce the desired cathodic reaction.
There are available in the marketplace at the present time various types of electrochemical sensors for sensing oxygen, for example, based on galvanic and potentiostatic principles which are utilized for determining oxygen concentrations with great accuracy over a wide range of concentrations ranging from 1 ppm (parts per million) to 100 percent oxygen without any problem. The measurements of gas concentration, such as oxygen concentrations at trace levels which are considered in the range of 1 to 100 ppb (parts per billion), is very difficult and has not been satisfactorily solved by the workers in the art. The problems associated with such sensing devices for sensing such low levels of oxygen or other electroactive gas concentrations is complicated by the very low level of signal output that is derived from the present day sensing cells and by the electronic signal-to-noise ratio generated by the presently available electronic circuitry makes the processing of such output signals very difficult. The presently available sensors also inhere the problem of the effective removal of any dissolved oxygen in the electrolyte solution for the sensing cell, and which removal procedure has been proven to be difficult and also requiring a long period of time to minimize the dissolved oxygen in the electrolyte. The residual oxygen dissolved in the electrolyte solution for such sensors therefore interferes with the correct sensing and measurement of the oxygen in a gas mixture. These problems are greater factors when the gas to be analyzed contains only a few parts per billion, ppb, of oxygen, or the like, of electrochemically active gas.
It is of course recognized that the output signal provided by such an electrochemical sensor may in principle be increased by simply increasing the effective area of the sensing cathode electrode. When the area of the sensing cathode electrode is increased, it requires a relatively large volume of electrolyte to wet the large cathode electrode. The removal of oxygen from a large volume of electrolyte becomes extremely difficult and time consuming. This, then, severely restricts the overall size of any electrochemical oxygen sensor. At the present time when oxygen is dissolved in the electrolyte solution of such a sensor, it can be removed by either purging the electrolyte solution by an ultra pure inert gas or by an electrochemical reduction mechanism, or the combination of the two procedures. These procedures usually require several days to remove the dissolved oxygen from the electrolyte and once the undesired gas is purged from the electrolyte solution, the electrochemical sensor still has to be calibrated by means of a sample gas with a known concentration of oxygen, or of the gas to be sensed. During the calibration process, some of the oxygen or other gas again dissolves in the electrolyte which has to be removed before using the gas analyzer to provide accurate results in analyzing a gas mixture having an unknown concentration of oxygen or the like. The amount of oxygen which dissolves in the electrolyte solution during calibration depends upon the extent to which the electrolyte is exposed to the calibration gas.
Another significant problem that is known in the art with respect to currently available gas sensors is that they generally exhibit a sensitivity to either the changes in the rate of flow of the gas undergoing analysis or to mechanical vibrations due to the vibration of the supporting surface for the cell causing stirring of the electrolyte, or to both. Any changes in the rate of flow of the gas undergoing analysis and the mechanical vibrations, both influence the rate of mass transport of the dissolved electrochemically active gas to the catalyst surface and, therefore, influences the accuracy of the output signal from the electrochemical cell. This requires the precise control of the rate of flow of the gas mixture and a vibration free supporting surface for the electrochemical cell for producing accurate measurements of the gas concentrations in a gas mixture.
At the present time, semiconductor devices have been extensively used in electronic circuits, including electronic circuit boards. The use of semiconductor devices is continuously increasing. During the fabrication of the semiconductor devices, ultrapure, inert gases are required for blanketing the semiconductor devices. It has been found that any oxygen in the blanketing gases is a major contaminent of the semiconductor devices and poses a significant problem in the fabrication of such semiconductor devices. The yield in the fabrication of such semiconductor products is adversely affected by the oxygen present in the inert blanketing gas. The semiconductor fabricators, therefore, deem it essential to accurately monitor the very low levels of oxygen concentration present in the blanketing gas during the fabrication process. To achieve this aim, accurate oxygen analyzers with sensitivities in the range of 0 to 100 ppb (parts per billion), are required. The presently available oxygen sensors do not lend themselves readily to measure oxygen in this very low range. The present status of the art is that most of the electrochemical sensors useful for analyzing gas mixtures having less than 10 ppm of oxygen contain an aqueous electrolyte solution. Present day electrochemical cells function to cause the reactant gas, such as oxygen, to be first dissolved in the electrolyte solution and then it diffuses to the cathode surface and then is electrochemically reduced at the cathode electrode surface, causing an electronic current to flow, producing oxidation at the anode electrode and a current flow into the external sensing circuit that is connected between the cathode and the anode electrodes of the electrochemical sensor. The rate of flow of electric current can be easily measured, and since it is proportional to the concentration of oxygen present in the gas mixture, signals the sensed concentration.
In addition, the aforementioned problems of the prior art sensors in ordinary usage, are even more critical when an attempt is made to utilize these sensors for measuring oxygen in the range of 0 to 100 ppb. Also, the gases to be analyzed are generally very dry and, therefore, the water in the aqueous electrolyte solution is continuously lost in the form of water vapor in the use of the electrochemical cells of the prior art. Since most of the electrochemical cells are sensitive to the amount of water in the cell, it has been found necessary to humidify the gases prior to introducing them to the sensing cell, in order to minimize the loss of water from the sensing cell. Accordingly, there is a present need in the art for an improved, accurate, electrochemical cell capable of measuring concentrations of electrochemically active gases in a gas mixture and, in particular, oxygen, in the range of 0 to 100 ppb and one that is simple to construct and use and has a capability of readily removing any dissolved electrochemically active gases from the aqueous electrolyte and does not need sample gas humidification.